whirling noise behind instrument cluster on 2006 Ford Five Hundred

Irregular, infrequent whirling noise. Seems to happen more often when I am rushing around town, doing a lot of heavy accel/decel. rarely happens when I am driving for hours on the highway. Has been happening for 6 years, and is slowly getting louder, but not distractingly loud at this point. Instrument gauges are not visibly affected. Beginning of day or end of day does not seem to be a factor.